In the field of electrophotographic printing, contact charging/erasing of the photoreceptor, herein called the "photoconductor", "organic photoreceptor" ("OPR"), or "organic photoconductor" ("OPC") has several advantages compared to charging with a corona discharge device. Contact charging, such as with a roller, results in effective and uniform erase and charging of the photoconductor surface. Roller charging features high charge efficiency with relatively low power supply requirements and features compatibility with a high speed EP process. A roller charging system also features a small footprint and can be designed to operate reliably and with minimal print faults and defects.
A roller charging system has the health and environmental advantage of producing a low amount of ozone compared to a corona discharge device. However, the ozone concentration at the photoconductor surface during roller charging is higher than during corona charging. This relatively high ozone concentration in and around the nip between the roller and the photoconductor can cause degradation of the photoconductor, especially of the photoconductor release layer. Such release property degradation, and especially the cumulative effect of such degradation over thousands of print cycles, can result in poor release of the developed image from the photoconductor surface to the paper or other print media, and poor release of the residual toner during a subsequent photoconductor cleaning step.
A charging roller or other charging member may comprise a variety of roller designs, such as the conventional rollers known well in the art. Many conventional rollers are conductive elastic rollers having a single layer of electroconductive rubber fixed on a metal core. This rubber layer typically has conductive particles dispersed throughout to give it an appropriate volume resistivity. Alternative rollers include multiple-layer designs, such as those disclosed in Tanaka, et al. (U.S. Pat. No. 5,089,851). The multiple layers of Tanaka include an inner elastic layer, a middle electroconductive layer, and an outer resistive layer.
Supply of a voltage to the roller or other contact charging member can be done in various ways, which are well-known in the art. The voltage may result from a DC source, an AC source, or a DC and AC source. Nakamura et al (European Patent Application 0272072) discloses charging by forming a vibratory field between the charging member and the charge-receiving member, which may be accomplished by superimposing a DC voltage and an AC voltage.